Maybe I can blame loosing all the Brawls I played with MegaByte on my use of the X button to jump, the supposed Wavebird lag, or that defending with L and R requires pushing until the triggers click.
Actually, I think it's simple that I suck, but if I could design my own SSBB controller it would have the Y, X, B, A setup of the SNES controller and bidirectional digital triggers like rocker switches. If I had this controller I wouldn't have to move my right thumb as far too reach the C attacks or the GCN's four buttons. Defend would be pushing in on the rocker shoulder trigger like pulling your hand in front of your face in boxing. And the outside R and L on the rockers would be attacks in those directions.
This setup would be similar to the Classic controller, but with rocker shoulder triggers, and the Y and A buttons closer. I'd rather use the GCN WAVEBIRD because it feels more comfortable in my hands and I prefer controllers with handles for intense games. Not to mention the annoying cord on the Classic Controller.
Even if I had my own custom made controller I'd still probably suck at SSBB and be easily beat by MegaByte.